Overload indicator for directional receivers



a Patented Dec. 22, 1942 ovER oAD mpicA'mnron DIRECTIONAL n n RECEIVERS Christopher Sydney Cockerell, Danbury,,Eng-,

land, .assignortto Radio Corporationof Amer I,

ica, acorporation of Delaware Application July 5, 1939; Serial N.282,846

lnGreat Britain July.29,.1938. l

2' Claims. (own-3111).

This invention relates to radio direction findingreceivers andmore particularly to such receivers of that at present common kind which are not fitted with automatic volumecontrol, but are fitted with manually operable sensitivity control efiective upon a carrierstage or stagesof the receiver.

The invention is illustrated in and explained in connection with the accompanying drawing in which Fig. 1 is a graph illustrating; a receiver overloading eifect; Figs. 2 and 3 are schematic diagrams illustrating, respectively, the invention applied to an anode bend detector, and to a grid leak detector, Fig. 4is a schematicfdiagrarn'of the invention applied to a diode detector; and Fig. 5 is a; circuit diagram of a portion of a superheterodyne receiver employing the invention. Similar referencenumerals will be used. to indicate similar elements of the several figures.

It isffound when operating a receiver of the kind referred to that errorsof 180 are sometimes obtained and in order that the present invention may be the better understood,the reason for these occasional but very fserious errors will first be described; The characteristic curve connecting input carrier (abscissae'ywith audio frequency output (ordinates) of areceiverfofthe kind referred to may take any of a variety of different forms, depending upon the detail designin the receiver, but in general is (as shown in Figure 1) a curve of bumpedgform. The audio frequency output (ordinates) 'increases'with increase in input carrier strength (abscissae) until a certain input strength (B) is reached after which,- for further increases of input carrier strength, the output decreases. The fact that the curve turns over inthis way is a source of danger and isresponsible for the phenomenon of reversed sense, i. e. for the occasional errors to which reference has been made. It willbe appreciated that if the manually operable sensitivity control is so set that the input when receiving any particular station, increases beyond that corresponding to the crown of the characteristic curve, the sense readings obtained may well be 180 wrong.

The object of the present inventionis to avoid this defect and this object is achieved, according to the invention, by providing the receiver with warning means operable automatically if the carrier strength at a predetermined point in the receiver increases beyond a predetermined strength. i

One way of carrying out the invention is to utilize the change of anode currentof an anode cient to flash the discharge tube. a

bend or of a. grid leak detector to actuate a warning signal. Where adiode or similartype of detectoris used, a fraction of the D. C. component of the rectified current may be fed to the grid of a following valve andthe resulting change in anode current of this valve employed'to actuate a warning signal. 1 i

In the embodiments shown in Figures 2 and 3 There is shunted across aresistance R1 in the anode circuit ofan anode bend (Figure 2) or of a grid leak (Figure 3) detector valve V-this valve maybe and preferably is the normally provideddetector of the receivera neon or other glow discharge tube circuit. The dischargetube circuit includes a discharge tube S in series with a resistance R3. The series combination is shunted by a'condenser G1 and is in series with a further resistance R2 across the detector valve ancderesistance B1. In the caseof an anode bend detector (Figure 2) as the signal strength on the grid increases the valve V takes more current, the potential difference across the anode resistance R1 increases, and the condenser C1 charges up. Inthe case of a leaky grid detector (Figure 3) as the signal strength on the grid increases the valve'vl takes lesscurrent but the anode .potential rises and again thecondenser C1 charges up. In either case when the charge in thelcondenser C1 is suffici ent' to cause the glow discharge tube S to break down through the re sistance R3, the discharge tube S flashes, dis charging the condenser C1 which accordingly recharges until it again reaches a potential sum- Accordingly, the discharge tube flashes, or winks automatically so long as the signal input strength remains high. The circuit parameters are so chosen that if the carrier strength on the grid ,9f the detector increases beyond the safe predetermined maximum, the tube S starts to flash and continues to do so until the carrier level is reduced again to a safe value, e. g. by the operator resetting his manual sensitivity control.

It will. be noted that, in addition to giving a visual flashing warning, an audible warning will be heard in the normally provided telephones or loudspeaker due to the sudden stopping and starting of the discharge tube. The audio outputto the telephones or the like may be taken oif through condenser C: where the valve V is the normally provided detector of the receiver.

In the modification shown in Figure 4, a diode or oxide type of rectifier X is used. This rectifier, which again is preferably the receiver detector, feeds via the usual smoothing network SN into the usual load resistance R4. The live end of this load resistance is directly connectedi. e., not through the usual blocking condenser-to the grid G of a valve V1. The normally utilized audio output is taken from the anode A of the valve V1, the anode circuit of which includes the usual anode resistance R1. Directly connected to the anode of this valve is a resistance R2 in series with a condenser C1. The condenser C1 is shunted by a series combination of resistance R3 and neon or other discharge tube S. With this arrangement, since the live end of the rectifier load resistance R4 is directly connected to the grid G, a D. C. component of the rectified output from the rectifier is applied to the said grid. Accordingly,-the anode current of the valve, and therefore its anode potential, will be a function of the carrier strength. In this arrangement also the discharge tube-condenser-resistance winking circuit is arranged to wink when the carrier strength exceeds the predetermined safe maximum.

Where the receiver is to receive continuous waves, a signal from the normally provided local heterodyne may be fed into the detector, this signal being of such value that it will not, in the absence of a strong carrier, operate the warning circuit. Figure 5 shows a circuit which is, in main essentials, very like that of Figure 4. In Figure 5 there are two rectifiers X1, X2, the circuit being part of a superheterodyne receiver employing two local oscillators (not shown). Intermediate frequency output derived by mixing the received signals with the first local oscillations are applied at I. F. and oscillations from the second local oscillator are applied at 2 L0. The valve V1 in Figure 5 is shown as a tetrode but, from the point of view of this invention, the main difference between Figures 4 and 5 lies in the changed position of resistance R3 which is provided to prevent excessive initial current flow at the commencement of discharge of the condenser C1.

As will be appreciated, if the values of Cl and R2 are not properly chosen, the tube S will not wink i. e. it will light and remain alight, still acting as a warning though not so effective a warnin as when it winks.

The Warning device itself may take any of a variety of difierent forms; e. g., instead of a neon or other discharge tube, an electrical or mechanical relay actuating a warning lamp or buzzer may be used or a cathode ray tube, or other visual indicating electron discharge device may be used.

In practice an arrangement in accordance with this invention operates not only as a warning device but as a rough visual indicator, indicating within a comparatively small arc the position of the minimum signal direction. This fact is of advantage since it considerably speeds up the process of taking a bearing by giving a rapid rough indication, the exact position of the minimum signal position being thereafter ascertained by the usual telephones or loudspeakers as in the usual way.

I claim as my invention:

1. In an overload signal device for a radio receiver, a detector responsive to signals applied to said radio receiver to produce a changing output current with increased signal strength, an output impedance for said detector, a network connected with the detector output impedance responsive to an output current resulting from overloading signals applied to said detector including a capacitor and a resistor connected to said detector output impedance in series and having charge and discharge periods of a rate below the persistence of vision for potentials corresponding to said overloading signals, and a visual warning indicator connected to said network between said resistor and capacitor and arranged to indicate by intermittent visual signals the application of signals overloading said receiver.

2. In an overload signal device for a radio receiver, a detector responsive to signals applied to said radio receiver to produce a changing output current with increased signal strength, an output impedance for said detector, a network connected with the detector output impedance responsive to an output current resulting from overloading signals applied to said detector including a capacitor and a resistor connected to said detector output impedance in series and having charge and discharge periods of a rate below the persistence of vision for potentials corresponding to said overloading signals, and a gaseous discharge warning indicator connected to said network between said resistor and capacitor and arranged to indicate by intermittent visual signals the application of signals overloading said receiver.

CHRISTOPHER SYDNEY COCKERELL. 

